IMO, a series starring Phil Morris as the Martian Manhunter would be kick-ass. Think Shaft with superpowers. Some other possibilities that only guested on Smallville:

Black Canary

Zatanna

Angel Of Vengeance

Hawkman - Hawkman has lots of potential. To start with, he's immortal through reincarnation, so his story can take place throughout history. Plus he flies like a bird, with a feathery 20-foot wing span, medieval armor and weaponry, an organizer or superhero teams, and a responsibility to save the world from big bad evil somewhere between Superman and Buffy. I've obviously been thinking about this one.